1. Technical Field
The present invention relates to the field of jet stream cutting, and particularly to abrasive jet stream cutting, wherein a suspension of abrasive particles in a fluid medium is pumped under high pressure and at high velocity against the surface of a workpiece to effect cutting operations. Such operations are widely employed in cutting of metal sheet and plate in fabrication of useful articles.
2. Prior Art
Abrasive water jets have grown to be widely employed in cutting and machining operations, particularly with metal sheet and plates to effect rapid and economical cutting and related forming operations. Typical applications have been the cutting materials which are difficult to machine, such as stainless steels, titanium, nickel alloys, reinforced polymer composites, ceramics, glass, rock and the like. Such techniques are particularly advantageous to produce cutting action through very highly localized action at low average applied force, to effect cutting of such materials with minimal thermal stress or deformation, without the disruption of crystalline structure, and without delamination of composite materials.
To effect abrasive water jet cutting, a specialized nozzle assembly is employed to direct a coherent collimated high pressure stream through a small diameter orifice to form a jet. Typically, pressures of about 200 MPa (about 30,000 psi) and higher are applied to force the media through the nozzle orifice.
Typical nozzle assemblies are formed of abrasion resistant materials, such as tungsten carbide or Boride. The orifice itself may be formed of diamond or sapphire. Abrasive particles are added to the high speed stream of water exiting the nozzle orifice by directing the water stream through a "mixing tube" and introducing abrasive particles into the tube in the region between the exit of the stream from the orifice and its entry into the "mixing tube." The mixing tube, which is typically several inches in length, is a region of contained, extremely turbulent flow in which the relatively stationary or slow moving abrasive particles are accelerated and become entrained in the high speed water stream, which may have nozzle exit velocities as high as Mach 3. The entrainment process tends to disperse and decelerate the water stream while the abrasive particles collide with the tube wall and with each other.
Relatively wide kerfs result from the dispersed stream, energy is wasted, and the tube is rapidly worn, even when made from abrasion resistant materials, such as tungsten carbide or Boride and the like. Some studies have shown that as much as 70% of the abrasive particles are fractured before they reach the workpiece to be cut.
In recent developments, water jets without abrasives have been thickened with water soluble polymers, which aid in obtaining and maintaining coherent jet streams, in reducing the level of misting, splashing and the like. Somewhat narrower kerfs can be achieved. Operating pressures and velocities remain quite high.
It is also known to suspend particulate abrasives in water jets, ordinarily relying on the thickening effect of the water soluble polymers to act as a suspending agent in the system. The abrasive cuts with greater efficiency than the water alone or the water with a thickening agent, but introduces an entire new spectrum of difficulties.